Kelley — 74 — Mycotrophy 



Pine is successful when planted in grey sand of Australia (Cromer, 

 1935) ; but Laitakari, already cited, found mycorrhizae most 

 plentifully developed in moorland soils and least in sands. Pecan forms 

 spreading clusters of mycorrhizal short-roots in light or sandy soils 

 but fan-like clusters in firm-textured soils such as the heavy red clay 

 subsoils of northern Georgia (Woodroof, 1933) . Influence of tex- 

 ture in humus on mycorrhizal form v^as neatly described by Mangin 

 (1910) : In soil formed of leaves which are superimposed and com- 

 pressed the mycorrhizae are often distichous with their branches in 

 the same plane ; while in contact with debris of cupules and fruits 

 their form is more or less complicated and branches of the mycorrhiza 

 are oriented in all directions and more or less pelotonized, dependent 

 upon size of the space in which it develops. In duff the mycorrhizal 

 short-roots of Populus were found to be clustered into nodules while 

 in sand they were betuloid in type, being ordinarily dark in coloui 

 except where growth is renewed (Kelley, 1937). Where a layer of 

 humus overlaid clay, Kelley (1941) found seedling pine with mycor- 

 rhizal short-roots in the humus but rootlets that had penetrated into 

 the clay beneath were transformed into pseudomycorrhizae. A some- 

 what similar case was reported by Frank (1888) in that a beech 

 seedling had mycorrhizae in the upper layers of soil (to 20 cm. depth) 

 while deeper occurring roots were fungus-free. Frank also noticed 

 that roots were much more richly branched in humus while in poorer 

 soil layers the roots assumed an elongate form. Perhaps the latter 

 were pseudomycorrhizae, but that term and concept had not been 

 formulated in Frank's day. 



Soil Moisture: — Mycorrhizae are found only under optimum 

 conditions of moisture ; that is, optimum conditions during the period 

 of growth. Thus, mycorrhizae are absent from aquatic plants so far 

 as known, yet they may be found on marsh plants (Mason, Osborn), 

 or on semi-aquatics like Orysa (Peyronel, 1922). They seem absent 

 or less abundant under bog conditions where pseudomycorrhizae are 

 more frequently found but are common enough in meadows and in 

 moist ground in general. On the other hand, they may be found in 

 soil that is very dry and even on desert cacti (Asai, 1934, Johansen, 

 1931) but they are present under arid conditions as reserve organs 

 and were formed while moisture was ample for growth. But too much 

 drying may result in death of the mycorrhizae, and Paulson (1923) 

 described an interesting case from Epping Forest, England, where 

 an unusually dry summer killed the mycorrhizae, thus cutting off 

 supplies of nutrient to the trees and paving the way for entrance of 

 secondary parasites and saprophytes by which many of the birch trees 



